The invention relates to a method for controlling the cooking degree of lignocelluloses as well as the pulp digesting operation by means of a FTIR (Fourier transform infrared) spectrometer for monitoring the infrared absorption of cooking liquors.
Continuous direct measurement of lignin concentration in cooking liquors during digesting operation has not been possible so far. Parameters influencing the digestion of lignocelluloses are therefore commonly used for cooking control in pulp mill process control systems. These parameters are cooking temperature, duration of the cook, digester pressure as well as concentration and composition of the cooking liquor. Even in combined so-called feedback/ feed forward systems (Bylund, L.; Hagglund, S.; Thorsell, L.; Wallin, G.: On-line cooking liquor analyzer - A means for effective control of sulfite digesters. 1982 International Sulfite Pulping Conference, Toronto, Oct. 20-22, 1982, TAPPI Proceedings 285-292 ) the cooking degree can only indirectly be determined, and the resulting kappa number deviations of the pulps from a preset value are relatively high. Kappa number is a measure of the residual lignin content of the pulp.
For that reason investigations have been made for some time to examine the applicability of fluorescence spectroscopy (Bublitz, W. J.: Fluorescence of pulping liquors: A tool for digester control? TAPPI 64 (6 ), 73-76 (1981 ) ) as well as light- and ultraviolet-photometry (Helmke, D.: Untersuchungen uber die Eignung der UV-Absorptionsmessung von Bisulfitablaugen zur Aufschlusskontrolle bei Zellstoffkochungen. Dissertation, Universitat Hamburg, 1973 and Tikka, P.O.; Virkola, N.-E.: A new cooking analyzer for monitoring organic and inorganic substances. 1984 Pulping Conference, San Francisco, Nov. 12-14, 1984, TAPPI Proceedings 291-296 ) for the determination of lignin concentration in cooking liquors. It became evident that either no good correlation existed between kappa number of the pulps and the measured lignin concentration in the liquor at the end of the cooks or that the time spent on the exact measurement of lignin concentration in the cooking liquor was quite long.
The present invention provides a method which overcomes these disadvantages.